An Nd—Fe—B rare earth sintered magnet is known as the permanent magnet with the best performance, widely used in voice coil motors (VCMs) of hard disc drives, magnetic circuits for magnetic resonance imaging devices (MRIs) and so on. Furthermore, it is known that in the magnet, the internal structure has a microstructure in which a periphery of an Nd2Fe14B primary phase is surrounded by a thin Nd-rich sub-phase, and thereby a coercive field is generated and a high magnetic energy product is exhibited.
On the other hand, in the case of a sintered magnet being used in an actual motor or the like, in actuality, grinding is applied to obtain the final dimension and concentricity. However, in this case, owing to such phenomena as minute grinding cracks or oxidation, the Nd-rich phase of the magnet surface layer is damaged, and as a result, the magnetic properties of the magnet surface portion deteriorate to a small fraction of that within the magnet.
This phenomenon is pronounced particularly in a small magnet where a ratio of surface area to volume is large; for instance, in the case of a block magnet of a 10 mm cube with the (BH)max of 360 kJ/m3, when cut and ground to 1×1×2 mm, the (BH)max deteriorates to about 240 kJ/m3; that is, the magnetic properties intrinsic to the Nd—Fe—B rare earth permanent magnet cannot be obtained.
In order to improve such defects of the Nd—Fe—B sintered magnet, there is a proposal in which a layer denatured by mechanical working is removed by means of mechanical polishing and chemical polishing (for instance, patent document 1) . Also, another method is proposed in which the magnet surface is ground and then covered by a rare earth metal, followed by diffusion heat treatment (for instance, patent document 2). Also, there is a method in which a SmCo film is formed on the surface of an Nd—Fe—B magnet (for instance, patent document 3).
Patent document 1: JP-A-09-270310
Patent document 2: JP-A-62-74048 (JP-B-06-63086)
Patent document 3: JP-A-2001-93715